4-(Indol-3-ylmethyl)-4-hydroxy-glutamic acid (3-(1-amino-1,3-dicarboxy-3-hydroxy-butane-4-yl)-indole) (hereinbelow referred to as “monatin”) represented by the following structural formula (3) is present in roots of a plant Schlerochitom ilicifolius and is a particularly promising low-calorie sweetener because of its remarkably high sweetness intensity (JP 64-25757 A):
